Commutator motors of this type, because of the soft iron auxiliary poles that are provided, exhibit an inverse speed characteristic, which makes them suitable for use as drive motors for starters in motor vehicles or for blowers in vacuum cleaners.
In a known commutator motor of this type (U.S. Pat. No. 3,594,599), the soft iron auxiliary poles are secured with threaded bolts to the short-circuit ring, and the permanent-magnet poles are arrested by means of pressing springs on the side remote from the auxiliary poles, these pressing springs being secured to the short-circuit ring and pressing the permanent-magnet poles against the corresponding associated auxiliary pole. This kind of arresting of the permanent-magnet poles is necessary for positioning them, so that pole shifting cannot be caused by centrifugal and cross-field forces during commutator motor operation or by jarring when the commutator motor is not in operation, such as when it is being installed in the motor vehicle. Especially in commutator motors having a high number of poles, this kind of pole shifting would mean a wider range of deviation in terms of brush twisting. Yet deviations from optimal brush twisting lead to increased brush wear (at an overly large twisting angle), or in a worsening of efficiency (at too small a twisting angle). The fastening construction of the permanent-magnet poles as described in the known commutator motor is very expensive to assemble, however, and is unsuitable for large-scale mass production.